This relates to graphics processing.
The reduction of memory bandwidth is of utmost importance, as energy efficiency is the performance factor that weighs most heavily when choosing a graphics architecture. Transactions over memory buses may cost several orders of magnitude more than computation in terms of energy and latency.
Hence, it is a common approach to attempt to reduce bandwidth usage with a bit of computation, resulting in less bandwidth usage than the original amount. This is the case for all sorts of color buffer compression.
For buffer compression, each tile (e.g., a rectangular region of pixels) must succeed in compressing the original data down to some threshold level if it is to be useful. For example, a tile that uses 1024 bits in uncompressed form may need to compress down to 512 bits if anything is to be gained from the compression. Hence, the more tiles that succeed at compressing to those thresholds (512 bits in this example), the less bandwidth to memory is used, i.e., the compression rate is important here. There may be several different thresholds, e.g., compressing from 2048 bits down to multiples of 512 bits: 1536 bits, or 1024 bits, or 512 bits. Obviously, they should be prioritized starting at the lowest threshold (i.e. 512 in this case).
A typical color buffer compression algorithm may find the minimum color components in the tile, and then use as few bits as possible to encode the residuals relative to the minimum color component per channel. Those schemes are sometimes called offset compression methods. Graphics application program interfaces (APIs) (OpenGL and DirectX) require that the color buffer is lossless, i.e., no information may be lost. Before offset compression is done, the colors for all pixels in a tile may be transformed using a color transform that decorrelates the information into a luminance component, and two chrominance components. Such a transform is the lossless YCoCg-transform. Transactions for the color buffers (including reading from an already rendered render target (RT) using a texture sampler) usually amount to the majority of the bandwidth in a typical graphics application. Hence, it is very important to attempt to increase the success rate of the compression method.